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Item type:Publication, Access status: with files , Simultaneous optimization of composite shells, metallic components and their jointsThis paper presents an approach for the simultaneous topology optimization of an isotropic (e.g. metallic) component, the layup optimization of a composite part and the search for the optimized position of joints between the parts. Within a gradient-based optimization, a failure criterion for the composite acts as a constraint. The constraint's gradients with respect to fiber orientations, pseudo densities of the isotropic elements, and joint positions are presented. As a demonstrative example, the load introduction into an isotropic component attached to a composite plate is analyzed. The results reveal a significant influence of the considered constraint on the optimized overall design.Publicationtype: Journal Article38 33 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Manufacture, process simulation, modelling and testing of thick-walled thermoset fibre-polymer composite laminates — a reviewThick-walled thermoset fibre-reinforced polymer (FRP) composites present unique challenges across their manufacturing, simulation, modelling, and testing processes. This paper provides a comprehensive overview of the current challenges and research needs associated with thick-walled FRP, particularly in light of their growing relevance in demanding application domains, such as wind energy. It is important to emphasise that the designation of a laminate as thick-walled is determined not solely by its nominal thickness, but also by the direction of the applied load. In particular, laminates subjected to compressive loading are typically considered thick-walled from a wall thickness of 4 mm or greater. While conventional manufacturing techniques remain applicable to thick-walled FRPs, process adaptations, such as adjusted curing cycles or alternative curing methods, are necessary to mitigate manufacturing defects, e.g. residual stresses induced by inhomogeneous curing due to local temperature overshoot. Modelling of the curing process and accurate prediction of residual stress development remain key areas of ongoing research with significant gaps in understanding. The influence of the wall thickness can also be seen in quasi-static and impact tests. Self-heating must be taken into account in fatigue tests and must be incorporated into future guidelines for the design of thick-walled FRP structures. While well-established non-destructive testing (NDT) techniques are generally applicable, their effectiveness is reduced with increasing laminate thickness due to limitations in resolution. The findings underscore the need for continued interdisciplinary efforts to refine processing and evaluation methods for thick-walled FRP composites.Publicationtype: Review Article73 28 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Automated composite design generation and optimization using variable angle tow plies for aerospace applications(Elsevier, 2025-07-31); ; In the aerospace sector, weight reduction is crucial for enhancing performance and efficiency. Significant advantages of using Carbon Fiber Reinforced Polymer (CFRP) in structural designs include the weight saving potential by leveraging their favorable strength-to-weight and stiffness-to-weight ratios, as well as their ability to modulate properties through strategic laminate design. Traditional composite design methods often depend on constant-stiffness, utilizing quasi-isotropic layouts with uniform fiber orientation angles across each layer. Although this method preserves structural integrity, it does not fully leverage the potential of advanced composite materials and fiber placement technologies for weight minimization. Introducing variable fiber directions can achieve further weight reductions. However, this expanded design flexibility introduces new challenges and necessitates automation in the design process. This paper builds on these concepts, presenting a novel approach to structural design using Variable Angle Tow (VAT) laminates for Automated Fiber Placement (AFP) manufacturing. Specifically tailored for complex double-curved components, this method aims to optimize weight and reduce manual effort while simultaneously maintaining or enhancing structural performance and ensuring manufacturability. It encompasses an automated nested loop optimization process utilizing Finite Element Analysis (FEA) and algorithmic design to fine-tune the orientation and curvature of splines, as well as the thickness of individual layers, thereby achieving mass reduction. The entire process effectively bridges the gap between design and manufacturing, from the initial CAD model import of the part surface to the generation of executable robot control code. The method is applied to a 2D plate and a double-curved 3D wing shell under exemplary loading conditions. Weight reductions of up to 7.44% were achieved compared to classical laminates. These exemplary results, while dependent on specific components and load cases, demonstrate the efficacy of the method and the potential for weight savings in aerospace structures.Publicationtype: Journal Article44 33 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , The influence of R/t-ratio on the imperfection sensitivity of the buckling load of thin-walled CFRP cylindrical shells(Elsevier, 2024-08-01); ; ; ; Thin-walled cylindrical shells under axial compression load are prone to buckling. The influencing factors leading to a reduction of the buckling load are a major design challenge. The sensitivity to imperfection increases with rising slenderness. In literature, cylinders with the same parameters and different slenderness are rarely tested in a larger number of samples. In this contribution, seven cylindrical shells, six of them nominally identical, are tested on two different test rigs and compared with twelve specimens with a smaller radius and same wall thickness from a previous series of tests. It is shown how the imperfection sensitivity is dependent on the R/t-ratio. Additionally, it is demonstrated how the quality of the test set ups corresponds with the achieved buckling load. Finally, the suitability of selected design approaches is determined based on the test results. Based on that, recommendations for the choice of a suitable approach are given.Publicationtype: Journal Article53 105 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Numerical investigation of the bearing performance of Thin- and Thick-Ply hybrid laminates(Elsevier, 2024-07-09); ; ; ; This publication presents a numerical investigation of Thin-Ply Hybrid CFRP bolted joints. In addition to Thin- and Thick-Ply specimens, hybrid specimens are investigated by substituting a part of the 90° CFRP layers with stainless steel foils. The numerical investigations are based on preliminary experimental work, which showed that fibre kinking led to final failure. A modelling strategy that includes 3D stress states and fibre kinking is chosen. The influence of micro-damages such as matrix cracks decreases with decreasing layer thickness. Therefore, and to avoid high computational power, primarily micro-damages such as matrix cracks are not included. The results of the simulations of the Thin-Ply and the hybrid samples show a good agreement with the experimental results. By hybridisation, the bearing performance can be significantly increased. It is shown that the modelling approach of not including micro-damage is well applicable for thinner plies but reaches its limits for thicker plies.Publicationtype: Journal Article34 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Microstructure generation and full-field multi-scale analyses for short fiber reinforced thermoplastics : application to PA66GF compositesShort Fiber Reinforced Thermoplastics (SFRTs) like PA66GF composites are highly heterogeneous materials with a rather complex microstructure such that the characterization and the prediction of their mechanical behavior remain quite challenging. So far, most of the research efforts have employed phenomenological and mean-field multi-scale models to deal with SFRTs. The present contribution rather focuses on a full-field multi-scale approach that incorporates advanced techniques in terms of microstructural representation and material modeling, allowing a deep insight of the dominating deformation mechanisms occurring in PA66GF composites. The proposed approach is based on an automatic periodic mesh generation algorithm for matrix-inclusion Representative Volume Elements (RVE) with randomly positioned fibers that follow a given Orientation Distribution Function (ODF). At the microscopic scale, while the fibers are assumed to be elastic, the behavior of the thermoplastic matrix is described by a phenomenological multi-mechanism constitutive model accounting for viscoelasticity, viscoplasticity and ductile damage. It results an advanced multi-scale model that enables to visualize the local deformation and degradation mechanisms occurring at the microscopic scale while simultaneously analyzing their influence on the macroscopic response of the composite upon monotonic, persistent and cyclic loading. The potential of the proposed approach is further evaluated by comparing the predicted responses against experimental data.Publicationtype: Journal Article52 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Probabilistic analysis of composite stiffened panels including random AFP-produced defects towards enhanced design allowables(2023-06-01); ; ; ; Automated fiber placement introduces defects such as gaps and overlaps during production, which add uncertainty to the mechanical behavior of composite parts. This study describes a probabilistic methodology which allows the direct introduction of random defects to the composite part and the calculation of the allowable design values through probabilistic virtual testing. The method is applied on a stiffened panel and is based on measurements regarding the random defect magnitude. An offline database of knock-down factors for stiffness and strength properties is first generated via detailed defected FE models. The factors are then assigned to the panel FE model accordingly, and via a Monte Carlo procedure the statistical response sample of the part is calculated. The process is further accelerated by a machine learning technique able to emulate the relationship between the random input and the selected responses. Several parametric analyses reveal that the number of defects on the panel is often more sensitive than the defect size, while the intensity of the defect at the through-the-thickness direction and the affected plies have a rather strong effect. Moreover, the conservatism of the deterministic design approach based on safety factors is proven by a direct comparison to the described probabilistic approach.Publicationtype: Journal Article70 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Health monitoring of CFRP laminates under cyclic loading via vibro-acoustic modulation based measurements(Elsevier, 2023-01-18); ; ; ; Estimating the damage state of composites is essential since carbon fibre reinforced polymers (CFRP) have a complex damage and propagation behaviour. Accordingly, a comprehensive study of the effects of matrix cracks – e.g. their size and crack density – and the influence of different damage types and their interaction on the vibro-acoustic modulation (VAM) behaviour of CFRP specimens under fatigue loading was carried out. These effects were evaluated in two sets of commonly employed laminate layups. Firstly, in a cross-ply (CP) layup, the number of 90°layers was varied, with a constant areal weight (268 g/m2). Secondly, quasi-isotropic (QI) specimens were tested, where the areal weight was reduced from thick-ply to thin-ply (30–268 g/m2), changing the failure mode from a delamination-dominated to a brittle failure. After damage introduction under tension–tension loading (R=0.1), high-frequency Lamb waves are introduced via piezoceramic actuators and modulated by a low-frequency vibration using a servo-hydraulic testing machine. VAM correlates with the different damage behaviour and damage conditions in CFRP. For this purpose, the evolution of the sideband amplitudes, the modulation index, the signal in time domain and X-ray scans are used. Regardless of the layup, the degree of stiffness-reducing inter-fibre fractures and large-scale delaminations can be detected.Publicationtype: Journal Article42 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , The influence of the manufacturing process and test boundary conditions on the buckling load of thin-walled cylindrical CFRP shells(Elsevier, 2023-01-06); ; ; ; Thin-walled cylindrical CFRP shells tend to buckle under compressive loads. The experimental buckling load is significantly lower than the theoretical one. The reason for this are various kinds of imperfections which are partially caused by the manufacturing process and the test conditions. A major challenge in the design of these shells is the lack of experimental data. Additionally, the quality of test results is rarely critically reviewed in the validation of design approaches. In this paper, the influence of two different manufacturing processes on the geometric imperfections and the influence of three different test rigs on the buckling load are investigated in an experimental campaign with 12 cylindrical CFRP shells. Furthermore, the influence of varying the test rig boundary conditions is analysed.Publicationtype: Journal ArticleScopus© Citations 4 80 178 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Weak adhesion detection – enhancing the analysis of vibroacoustic modulation by machine learningAdhesive bonding is a well-established technique for composite materials. Despite advanced surface treatments and preparations, surface contamination and application errors still occur, resulting in localised areas with a reduced adhesion. The dramatic reduction of the bond strength limits the applicability of adhesive bonds and hampers further industrial adaptation. This study aims to detect weak-bonds due to manufacturing errors or contamination by analysing and interpreting the vibroacoustic modulation signals with the aid of machine learning. An ultrasonic signal is introduced into the specimen by a piezoceramic actuator and modulated through a low frequency vibration excited by a servo-hydraulic testing system. Tested samples are single-lap shear specimens, according to ASTM D5868-01, with artificial circular debonding areas introduced as PTFE-films or a release agent contamination. It is shown that an artificial neural network can identify various defects in the bonded joint robustly and is able to predict residual strengths and hence demonstrates great potential for non-destructive testing of adhesive joints.Publicationtype: Journal ArticleScopus© Citations 15 230 344